During the operation of motor vehicle generators, for instance claw pole generators, sudden surge voltages occur in response to a load dump because the magnetomotive force or the stored magnetic energy of the field winding of the generator can only be degraded at a finite rate. Traditionally, the resultant voltage increase in the vehicle electrical system can only be prevented by using additional, expensive components, such as power zener diodes.
Various circuits for quenching or degrading the stored magnetic energy of a field winding are known from the related art.
We will first describe one known quenching circuit with reference to FIG. 3. A generator G is furnished with a field winding 10 that is characterized by its inductor L and its ohmic resistor R. The storage or quenching of energy of field winding 10 is controlled by a power transistor T, which in normal operation is operated by timed pulses from a clock control 20. Field coil 10 and power transistor T are connected in series to a battery voltage Ubatt.
As mentioned above, in normal operation, power transistor T is operated by timed pulses, for instance by the process of pulse width modulation. During the conductive state of power transistor T, energy is stored in field coil 10. A diode D makes a free-running circuit of field coil 10. During the non-conducting state of power transistor T, i.e. the free-running phase of field coil 10, the magnetic energy stored in the field coil is degraded via the free-running circuit. The counter-voltage required for the energy degradation corresponds to the conducting-state voltage of diode D of the free-running circuit, which is typically about 0.7 V. The degradation of energy proceeds relatively slowly, corresponding to this low counter-voltage. While this behavior is desirable in normal operation, in the event of a load dump, the energy degradation in the field coil proceeds too slowly, despite immediate intervention by the regulator, because the low counter-voltage does not permit rapid degradation of energy. This leads to surge voltages in the vehicle electrical system which must be suppressed, as for example by the use of expensive power zener diodes.
FIG. 4 shows a so-called H-bridge circuit for degradation of the energy of a field coil, which is known from the related art. The components illustrated here have the same reference numbers as those in FIG. 3. The circuit has two power switches S.sub.1, S.sub.2 that can take the form, for example, of transistors. In normal operation, both power switches S.sub.1, S.sub.2 are closed for storage of energy in field winding 10. In this case, current flows via power switch S.sub.1, inductor L, resistor R and power switch S.sub.2. During the free-running, one of the power switches is closed, which facilitates a degradation of the magnetic energy of field winding 10 according to the design approach described above with reference to FIG. 3.
For rapid degradation or quenching of the magnetic energy of field winding 10, both power switches S.sub.1, S.sub.2 are opened simultaneously. The current now flows via diode D.sub.2, inductor L, resistor R and diode D.sub.1. In this case, diodes D.sub.1, D.sub.2 are traversed by the flow in the direction of conductance. Hence the current flows against the battery voltage (for example, 14 V) and the conducting-state voltages of the two diodes D.sub.1, D.sub.2. The counter-voltage required for this is built up by the self-inductance of field coil 10.